I've got a very strange problem with the application I create for G55 display. The display has CO detector (supports Modbus) connected to the COM1 port. Pin 2 of the serial port connector at my G55 (which is Tx pin) is connected to the Rx contact of the CO detector; pin 3 (Rx) is connected to the Tx at the detector, pin 9 is a power connection to the power supply, and pin 5 is a ground at power supply. CO detector is powered from its own source. COM port settings at both devices are standard ones: baud rate - 9600; data bits - 8; parity - no; stop bits - 1.

I created very simple application to test connection with just MBComV2 component and MBRegisterV2 component using that connection. MBCommV2 has everything by default (SerialPort = COM1; ConnectionType = mb_Serial; ConnectAtStartup = True). MBRegisterV2 has ComBox = ModbusComm; VarType = mb_int16. But when I upload the application to the G55, I get no connection with the CO detector. And now I need an advice what could cause the problem here? If I connect that cable to COM port at my laptop, I can communicate to CO detector using its own application. So, looks like the problem is somewhere at the display side... I tried reducing baud rate for connection, tried add/remove ground wire between CO detector and G55, tried switching Tx and Rx wires, but all with the same result. And now I have no more ideas how to make it working....

Regards,Alexey.

Update: the device I am trying to communicate with is Scott Instruments 4600MB gas detector.

Assuming you have a panel mount G55 and looking at the documentation that sounds correct. Pin 2 is TX and pin 3 is RX on the G55 DB9.

Yes, that is a panel mount G55.

Ron L. wrote:

" And now I need an advice what could cause the problem here? "

What are the communication error messages you are seeing?

Unfortunately, I can't tell right now, but will be able to answer later today or Monday.

Ron L. wrote:

This sounds like a cable problem Are both sides DB9's? If so there should be no customer cabling needed you can just use straight through or null modem.

No. One side of the cable is DB9 (which is a G55 side), but the other one is a terminal blok on the gas detector board. So, I use the custom cable instead of a standard DB9 to DB9 one. But I don't think it is a wiring problem: the same cable is used to connect the gas detector to the laptop, and it works.

Also, while I'm waiting for my gas detector being returned, a question. Are RTS/CTS necessary for G55 to perform communication? Why do I ask - these pins are not connected now. Could it make problems in communication?

No. One side of the cable is DB9 (which is a G55 side), but the other one is a terminal blok on the gas detector board. So, I use the custom cable instead of a standard DB9 to DB9 one. But I don't think it is a wiring problem: the same cable is used to connect the gas detector to the laptop, and it works.

That sounds like exactly the problem. A cable that can successfully connect your gas detector to the PC would need to go through a null modem in order to talk with the G55.

No. One side of the cable is DB9 (which is a G55 side), but the other one is a terminal blok on the gas detector board. So, I use the custom cable instead of a standard DB9 to DB9 one. But I don't think it is a wiring problem: the same cable is used to connect the gas detector to the laptop, and it works.

That sounds like exactly the problem. A cable that can successfully connect your gas detector to the PC would need to go through a null modem in order to talk with the G55.

Another words, I need to switch Pin2 and Pin3 wires, right? But I tested that configuration as well, and it didn't work...

There's a limited number of things that could be wrong. Here are the possibilities.

1. The settings on the G55 do not match the serial settings on the other device it's talking to.2. The cable is the problem.. Since both sides are RS232 with DB9, there is not need for problems here because you can purchase pre-made null-modem cables.3. The G55 serial port is not working. You can test this by downloading to the terminal over serial (if it's COM1). If the G55 serial port hardware is broken you can send it for repair, but there will be a charge if no problem is found or if it's out of warranty. Here is the return form.http://www.beijerelectronicsinc.com/sup ... equest.php4. The device the G55 is talking to talk to at a hardware level is not very robust is it's serial communication for some reason only works with PC serial ports. Hard to prove or dis-prove this without some electrical debugging of the serial communication lines using an oscilloscope.

Looks like I figured out the problem. It was the ground contact. As I said above, G55 and gas detector had separate power supplies and thus different ground contacts. But when I connected both devices to the same ground to let both devices compare the signal to the same zero, everything started to work perfectly. Ron, Jeremy, thanks for your time and help!

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